The present invention relates in general to intake manifolds for internal combustion engines. More in particular, it relates to a unique two-plane, independent runner intake manifold of high volumetric efficiency.
A carbureted internal combustion engine employs an intake manifold to distribute a fuel and an air mixture produced by a carburetor to the cylinders of the engine.
An intake manifold typically has a plenum chamber below the carburetor to receive a mixture of fuel and air from the carburetor. From the plenum the mixture travels to the cylinders through ducts called runners. The runners exit from the manifold at inlet ports to the engine. These ports lead to the cylinders through inlet valves. The inlet valves open and close every other revolution in a four-cycle engine, and do so gradually, that is, the valves do not open and close instantaneously.
A fuel and air mixture is drawn into each cylinder of an engine by a vacuum created there by downward piston movement during the intake stroke of the cylinder. Inlet and exhaust valves into each cylinder provide for the admission of the fuel-air mixture into the cylinder and the exhaustion of products of combustion from the cylinder. The engine itself provides the power to induct the mixture it burns for power. At its lowermost position in a cylinder, a piston is at bottom dead center. The uppermost position of a piston is top dead center.
The dynamics of induction of fuel and air into an engine are very complicated, making generalization difficult. Factors affecting induction include intake and exhaust valve timing, piston speed, inertia of gases undergoing induction, fluid friction, resonance, intercylinder interference, and induction geometry, to name only a few.
The intake valve timing of today's internal combustion engines has the inlet valve opening while the exhaust valve is closing, but before the piston reaches top dead center. Inlet valves close several crank degrees after bottom dead center. This timing accommodates the fact that it takes several crank degrees to effectively open and close the inlet valve. In other words, to have the inlet valve as open as much as possible during the descent of the piston, the inlet valve is given a head start and starts to open before the piston actually begins to descend. To have the inlet valve open and to take advantage of gas inertia, the inlet valve does not close until the piston has begun ascending in the cylinder again. It is quite apparent that the more mixture that is inducted into a cylinder with each cycle, the more efficient the engine will be. If the gases flowing in the runners of an inlet manifold are flowing at a relatively high velocity their inertia can result in an additional amount of mixture charged into the cylinder.
Piston speed directly measures the pumping characteristics of an engine. The higher the piston speed the more mixture is inducted into the engine in a unit of time. Second, piston speed generates pressure pulses that affect movement of the mixture in the intake manifold. As the piston descends, a negative pressure pulse results and this pressure pulse travels upstream of the manifold. As the piston ascends, a positive pressure pulse results and again the pressure travels upstream in the inlet manifold. It is pressure differential that results in mixture movement and the pulses form components of the differential. The pressure pulses travel at the speed of sound. Gas velocity is much slower. The pressure pulses can be used to enhance volumetric efficiency. As a pressure pulse travels up a runner of a manifold and reaches atmosphere, which may be the plenum of the manifold, the gas there overcompensates for the disturbance caused by pulse. Thus when the pressure pulse traveling upstream is positive with respect to the mean inlet manifold pressure it pushes air out of the way in the plenum and creates a locally rarified zone. Rarefaction results from the inertia of the air responding to the positive pulse forcing air out of the zone. The resulting negative pressure travels down the manifold and obviously affects the flow of gas in the manifold. Of more interest is the negative pressure pulse that travels upstream in response to a descending piston. This negative pressure pulse will create rarefaction in the plenum and the gas will rush in to fill the resultant depressed zone generating a positive pressure pulse that travels down the runner towards the cylinder. If the pulse arrives at the cylinder at the right time, say when the inlet valve is about to close, the pulse can add significant quantities of mixture to the cylinder to increase the power of the engine by increasing the volumetric efficiency of the engine. This is known as intake manifold tuning and obviously relies upon the resonance of the mixture which, practically speaking, means resonance of the air.
The speed of the pressure pulse is largely independent of manifold geometry. The velocity of the gas however is not. As the cross-sectional area of the manifold runners decreases, the gas increases in velocity. As the length of the runner increases, the time required for a pulse to travel upstream and back downstream increases. Increased mixture velocity improves the opportunity for cylinder filling at the end portions of the induction cycle. Torque of an engine at relatively low engine speeds is enhanced by increasing the velocity of the mixture in the intake manifold. Fluid friction can become a problem, however, when gas velocity is increased too much.
The pressure history of one cylinder in a multiple cylinder engine can affect the induction performance in other cylinders. Thus pressure pulses traveling up a runner from one cylinder can interfere with the pressure within other runners. Generally it has been the practice to design inlet manifolds with a view towards the elimination of this intercylinder interference.
One of the most popular manifolds produced in this country is the so-called two-plane, over and under, 180.degree. manifold. This manifold has been a standard for some time for most American production V-8 engines using a single 4-barrel or 2-barrel carburetor. The manifold has runners disposed in a complex array. The plenum does not directly communicate with each of the manifold runners. Instead, stubs between the runners and the manifold communicate two or more of the runners with the plenum. The idea behind the two-plane manifold is to isolate cylinders of an engine so that there will be little or no intercylinder interference. The manifold has two plenums, one over the other. Alternate runners, in the sense of the engine's firing order, go to alternate of the plenums. Physical separation into two plenums contains pressure pulses.
In an independent runner manifold, such as described in U.S. Pat. No. 3,744,463 to James McFarland, a common plenum for all of the cylinders of an engine directly communicates with the cylinders of the engine through an independent runner for each cylinder. Thus, for a V-8 engine there are eight independent runners with no stub passages shared by two or more runners. The plenum employs no partitions to separate the plenum into two plenums. Independent runner manifolds in many applications have advantages over the two-plane manifolds. The advantages inhere from simpler induction paths afforded by the manifold and include better cylinder-to-cylinder air-to-fuel ratio uniformity and lower pumping friction work.
A second type of independent runner manifold that does employ compartments in a plenum is described in U.S. Pat. No. 2,771,863 to Ferdinand Porsche. The manifold of the Porsche patent discloses two side-by-side plenums not in communication with one another. The runners leave the plenums at the same elevation. The runners radiate from the plenums with runners to one side of the engine coming from one side of the manifold while runners to the other side of the engine come from the other side of the manifold.